The present invention relates to a balancing method for a rotor which is driven at a speed near or above its critical speed, i.e., a flexible rotor and a balancer applicable to the balancing method.
A conventional balancing method for a flexible rotor will be described with reference to FIGS. 1 to 3.
A rotor shown in FIG. 1 resonates when its rotational speed is increased to a level near its natural frequency, so that it exhibits a bending mode as shown by an imaginary line in FIG. 1. When this is shown by a response amplitude curve, the phenomenon is shown by a curve No. 1 in FIG. 2 and when by a polar coordinate representation of the bend mode in the form of a vibration vector determined by response amplitude A and a phase .theta., it is shown by a curve No. 1 in FIG. 3.
Then, the rotor is stopped and disassembled, and a test weight Wt is attached to the rotor. The rotor is then assembled and operated again for the measurement of vibration. It is assumed here that curves denoted by No. 2 in FIGS. 2 and 3 are obtained as the response amplitude and vibration vector. Then, influence coefficient indicative of the influence of the test weight Wt is determine based on the curves No. 1 and curves No. 2 in FIGS. 2 and 3, and the position and mass of a correction weight Wc is determined on the basis of the influence coefficient.
Then, the rotor is stopped and disassembled, and the correction weight Wc determined as above is attached to the rotor. The rotor is then assembled and operated again for the purpose of confirmation of the effect of the correction weight Wc. If the response amplitude and vibration vector as shown in FIGS. 2 and 3 by curves No. 3 are obtained, it is decided that the correction weight Wc has been determined correctly.
The described process, however, is an ideal one. Actually, operation with correction weight often fails to provide desired vibration characteristics. Consequently, steps have to be repeatedly executed for determining the correction weight, until the desired vibration characteristic is obtained.
The described balancing method is a method considering only correction plane to which a correction weight is mounted. When there are a plurality of correction planes, it is necessary to determine the influence coefficient for each of such correction planes. For instance, when there are three correction planes, operation with the test weight has to be conducted three times.
The described process is for obtaining balance against first order mode resonance shown in FIG. 1. An impractically large number of steps, requiring much time and labor, are necessary when balance has to be attained for resonance of higher orders such as resonance of second order or ternary mode.
Meanwhile, many reports have been made regarding suppression of rotor resonance by electrial means. For instance, an article of Kanamitsu et al., PROCEEDING OF 3rd INTERNATIONAL CONFERENCE ON ROTOR-DYNAMICS (1990.9.10-12, LYON-FRANCE), pp 263-268, discloses that vibration of a rotor carried by magnetic bearings can be reduced by application of a vibration exciting force synchronous with the rotor rotation from the magnetic bearings to the rotor and that the vibration is increased again when this external vibration exciting force is nullified.
This method, however, is applicable only to rotors which are supported by active type bearings such as magnetic bearings but is not applicable to rotors supported by passive type bearings such as slide bearings, ball bearings or oil film bearings.